The present invention relates to a wire pit. Especially preferably the invention relates to a new kind of wire pit construction having a wall/walls converging downwards so that the average flow direction of the liquid at the most part of the wire pit""s height deviates from vertical.
Almost all prior art paper machine approach systems feeding paper pulp to the paper machine, which are well described e.g. in U.S. Pat. No. 4,219,340, comprise the following components: A white water tank, a centrifugal cleaning plant with feed pumps and pumps between various stages, a gas-separation tank with vacuum providing means, a head box feed pump, a head box screen, a paper machine head box and white water trays. Said components are placed in connection with the paper machine and arranged to operate as is follows. The fiber material used for paper making and the fillers which are diluted with so-called white water obtained from the wire section of the paper machine are dosed by means of a basis weight regulation valve from the machine chest into the white water tank usually located at the bottom level of the mill. By means of a feed pump also located at the bottom level of the mill, the fiber suspension is pumped from the white water tank to the first cleaning stage of a centrifugal cleaning plant usually located at the machine level, i.e. the location level of the paper machine, or, as in said patent, above it. The centrifugal cleaning plant most typically comprises several (most commonly 4-6) stages each typically having a feed pump of its own. By means of pressure created by said feed pump, the fiber suspension accepted in the first cleaning stage of the centrifugal cleaning plant is further conveyed to a gas-separation tank typically located at a level above the machine level. In the gas-separation tank, the fiber suspension is subjected to the effect of vacuum created by means of vacuum providing apparatus, most usually liquid rings pumps, whereby both part of the gas dissolved in the suspension and the gas existing in the suspension in small bubbles rises above the surface of the liquid in the tank and is discharged from the tank via the vacuum providing apparatus. From the gas-separation tank the fiber suspension, wherefrom gas has been removed as thoroughly as possible, flows to a head box feed pump located at the bottom level of the mill, which pump further pumps the fiber suspension to a head box screen (not shown in said U.S. patent) also located at the bottom level of the mill, whereafter the fiber suspension flows to the machine level into the head box of the paper machine.
One problem in the prior art paper machine approach system is its huge volume mostly due to the volume of the gas-separation tank and the centrifugal cleaning plant as well as the long and large-sized piping. Volume in itself is not a major problem, except for space requirement and being a relatively big investment, but long delays caused by great volumes substantially restrain the change of grade and result in great amounts of broke in connection with the changes of grade. In connection with the grade change, broke is formed of all the pulp being used to produce the final product before the relative amounts of all components of the fiber suspension have been equalized throughout the approach system to correspond to the content of the desired final product.
Said problem has already been dealt with in FI patent 89728, according to which different types of white waters are collected from the wire section of the paper machine and guided directly to the short circulation of the paper machine without employing any actual white water tank. In said publication, under each white water tray there is a pump for delivering the white water to a suitable location. The publication describes the white water channels to be very flat, i.e. of small volume, so that the delays remain as short as possible. In the solution according to said publication, arranged at the side of the wire section there is a small pumping container and means providing pumping operation, from which the white water is further delivered to the process. The deaeration reached by means of this apparatus is not efficient enough to provide undisturbed operation of the paper machine, though. In other words, despite the possibility of removing gas from white waters by means of a pumping device according to the publication, this has not succeeded to an extent allowing for eliminating the wire pit, i.e. the white water tank, assisting in the gas-separation.
Thus, despite progressive proposals in order to eliminate the wire pit, one still has to accept the presence of the wire pit in the paper machine approach system. Nevertheless, one does not have to accept the great height of the white water tank to be a reason to the energy consumption of pumping in the paper machine approach system. The white water tanks, into which the so-called white waters from the paper machine are collected, have traditionally been relatively big containers having a height of almost ten meters, located at the bottom level of the paper mill. The surface level of these tanks, although keeping constant in an individual tank mostly due to overflow, has been greatly altering in relation to the paper machine. One reason for the altering of the surface level is the location of the white water tank in connection with the machine. In case of a so-called fourdrinier machine, the white water tank, which in said case is also referred to as wire pit, is located below the wire section, whereby its surface level has been relatively low, due to e.g. constructional reasons. Also, the surface level of a white water tank arranged aside the wire section or the like (a so-called off-machine silo) is not always as high as it might in practice be. The big size of the white water tank has been justified on the basis that the presence of a big buffer tank has been considered a positive factor stabilizing the process. This has also caused both some extra energy consumption, as the first feed pump has had to compensate for the sometimes-low surface level of the white water tank, and additional delays in the process caused by the big volume of the white water tank.
In the way according to the FI application 981798, it is possible to avoid locating said white water tank in the paper machine approach system at the bottom level of the mill, i.e. below the machine level. The solutions described in said application allow for arranging the white water tank at the machine level, whereby the gas-separation tank feed pump aside the white water tank is also located at the machine level.
However, said publication mainly concentrates on the possibility of decreasing the energy consumption of pumping by utilizing a propeller pump located at the machine level. Said publication only mentions that at the same time it is also possible to decrease the height of the wire pit and accordingly decrease the time needed for changes of grade.
The present invention handles with problems related to the construction of a low wire pit and different factors, which have to be taken into account when designing a wire pit.
Firstly, as already stated above, the wire pit must operate as a vessel separating gas from the white waters, whereby the same rules that apply to other vessels used for gas-separation apply to the construction of the wire pit as well, that is the open liquid surface must be as large as possible. A good starting point may be e.g. that the cross-section of the wire pit is kept essentially unchanged.
Secondly, the liquid flow into the wire pit must be kept as laminar as possible in order not to disturb the gas-separation. Further, because various white waters, i.e. for example having various fiber contents, enter the wire pit, the liquids should have to be directed into the wire pit in such a way that the cleanest fraction of the white waters would be directed to the overflow of the wire pit.
Thirdly, both the liquid entering the wire pit and the liquid discharged therefrom should be as non-turbulent as possible, so that the turbulence would not hamper the separation of gases from the white waters and the mixing of thick pulp in the white water in the wire pit discharge.
In addition to that, as a wire pit of small volume is practically in every respect better and more practical than one of great volume, it should be possible to locate a small-volume wire pit in an older paper machine approach system as well, that is in an apparatus where both the mixing pump and the mixing of thick stock and chemicals are arranged at the bottom level of the mill. To put it differently, the ideal construction of a wire pit would be a construction that could be modified as easily as possible in relation to flows going to and from various locations and, in addition to that, arranged in new systems at the machine level of the mill and in older systems at the bottom level of the mill.
A preferable solution according to the present invention is a wire pit made of modules in such a way that its parts may be located in several various positions in relation to each other.
Characterizing features of the wire pit according to the invention are described in the appended claims.